1. Field of the Invention
The present invention relates to a color image forming apparatus such as a color copier or color printer that uses an electrophotographic method.
2. Description of the Related Art
A color image forming apparatus that forms a color image on a recording medium by sequentially superimposing toner images of the four colors of yellow, cyan, magenta and black on the recording medium is already known. In this kind of color image forming apparatus, if the position at which images of a plurality of colors are superposed deviates relatively between the colors, an error referred to as a “color deviation”, that is, positional deviation of image, occurs. Consequently, it is necessary to align the toner images of the respective colors that are superposed on the recording medium, as accurately as possible.
In recent years, there is a demand for image forming apparatuses to achieve faster printing and to reduce the time a user waits from when a print job is input until the first sheet is printed out. Although the actual operations for outputting a color image and the preparatory operations therefor had previously been carried out sequentially, in response to the aforementioned demands for faster printing and reduced waiting times, image forming apparatuses now execute control to perform such operations in parallel. As an example of an operation that is performed in parallel with a certain electrophotographic process operation, contact of a cleaning blade or a transfer unit against an intermediate transfer member or separation of the cleaning blade or transfer unit therefrom can be mentioned. However, a load change arises due to a mechanical shock that occurs when a cleaning blade or a transfer unit contacts against or is separated from contact with an intermediate transfer member, and a slight error arises in a peripheral length value that is detected. As a result, there is the problem that an error occurs with respect to the alignment of the toner images of each color and consequently a color deviation occurs. Therefore, Japanese Patent Application Laid-Open No. 2000-066475 discloses technology in which a correction value used for color deviation correction control is switched between a time of forming an image on a first sheet that does not receive the influence of contact or separation of a transfer unit or a cleaning blade, and a time of forming an image on a second or subsequent sheet does receive the influence of such contact or separation.
In this connection, in recent years the types of media (recording medium) that are used for printing by image forming apparatuses have become more diverse, and some image forming apparatuses include a plurality of image forming modes to correspond to a variety of media types. For example, such image forming apparatuses perform image forming by varying a fixing temperature or an image forming speed in accordance with respective image forming modes such as a plain paper mode, a thick paper mode, and a glossy paper mode. More specifically, there is a desire to suppress a color deviation that is caused by the above described mechanical shock in a manner that also corresponds to each of these image forming modes.